Controls the global menu status callback so a GLUT program can determine
when a menu is in use or not. When a menu status callback is registered, it
will be called with the value InUse when pop-up menus are in use by the
user; and the callback will be called with the value NotInUse when pop-up
menus are no longer in use. Additionally, the location in window coordinates
of the button press that caused the menu to go into use, or the location where
the menu was released (maybe outside the window). Other callbacks continue to
operate (except mouse motion callbacks) when pop-up menus are in use so the
menu status callback allows a program to suspend animation or other tasks
when menus are in use. The cascading and unmapping of sub-menus from an
initial pop-up menu does not generate menu status callbacks. There is a
single menu status callback for GLUT.

When the menu status callback is called, the current menu will be set to
the initial pop-up menu in both the InUse and NotInUse cases. The
current window will be set to the window from which the initial menu was
popped up from, also in both cases.

Idle callback

Controls the global idle callback so a GLUT program can perform background
processing tasks or continuous animation when window system events are not
being received. If enabled, the idle callback is continuously called when
events are not being received. The current window and current menu will
not be changed before the idle callback. Programs with multiple windows
and/or menus should explicitly set the current window and/or /current
menu/ and not rely on its current setting.

The amount of computation and rendering done in an idle callback should be
minimized to avoid affecting the program's interactive response. In general,
not more than a single frame of rendering should be done in an idle callback.

Timer callbacks

Register a one-shot timer callback to be triggered after at least the given
amount of time. Multiple timer callbacks at same or differing times may be
registered simultaneously. There is no support for canceling a registered
callback.

The number of milliseconds is a lower bound on the time before the callback
is generated. GLUT attempts to deliver the timer callback as soon as possible
after the expiration of the callback's time interval.